November 2-5, 2010, Boulder, Colorado

We are pleased to announce the second in a series of meetings convened by the MESSENGER and BepiColombo science teams. The purpose of this workshop is to prepare for MESSENGER orbital operations and to plan for the BepiColombo mission. The workshop is motivated by observations of Mercury’s magnetosphere and exosphere obtained during MESSENGER’s three Mercury flybys, which occurred in 2008 and 2009.

Before the MESSENGER flybys, ground-based observations of volatile species (Na and K) led to a picture in which thermal and solar radiation processes acted on the sunlit surface to continuously release material to the exosphere. Solar wind sputtering, modulated by the planet’s magnetic field, provided spatial and temporal variability. Observations of high-energy Ca could be explained by a two-step process in which CaO was released from the surface with relatively low energy and subsequently ionized. New space-based observations obtained during the MESSENGER flybys argue that this picture is incomplete and that the processes that generate and maintain the exosphere are more complex. Na and two refractory elements, Ca and Mg, all exhibit different spatial distributions over the poles and in the tail region that cannot be fit into the traditional model. These measurements as well as MESSENGER’s remote-sensing detection of Ca+ concentrated in the near-planet tail region point to an exosphere that is both more varied and perhaps more intertwined with the magnetospheric environment than previously thought.

BEPICOLOMBO spacecraft.
Image: ESA

Earlier Mariner 10 observations indicating the presence of a primarily dipolar planetary magnetic field oriented largely along Mercury’s spin axis, but 1000 times weaker that that of the Earth, have been confirmed by MESSENGER. However, Mercury’s magnetosphere has been found to be far more dynamic than that of the Earth, with the effect of reconnection between the interplanetary magnetic field and the planetary magnetic field proceeding at a rate about 10 times that observed at the terrestrial magnetopause. Initial examination of the MESSENGER flyby measurements indicates that this very strong interaction with the interplanetary magnetic field may greatly increase the flux of solar wind plasma to the surface, for example, by creating large magnetic fields normal to the magnetopause, giant flux transfer events, and the expansion of the high-latitude magnetospheric cusps. Furthermore, the discovery of an intense dayside boundary layer and nightside magnetic field depressions, both presumably due to hot solar wind or planetary ions, suggest the presence of strong plasma transport and heating processes. However, the lack of energetic particle events, even when the magnetic field shows that substorms are present (i.e., the third MESSENGER flyby), may require a re-examination of the candidate charged particle acceleration processes and the earlier Mariner 10 events.